The present invention relates to semiconductor packages with heat dissipating structures, and more particularly, to a semiconductor package having a heat sink whose surface is exposed to outside of a chip-encapsulation encapsulant so as to improve heat dissipating efficiency of the semiconductor package.
Generally in semiconductor packages, a molding resin or encapsulation compound used for encapsulating chips is mostly a material having poor thermal conductivity such as epoxy resin, making heat produced from operation of the chips not able to be effectively dissipated via the molding resin to the atmosphere, which may thereby degrade reliability of the semiconductor packages especially for those mounted with highly integrated chips therein.
For solving the above heat dissipation problem, it is desired to incorporate a heat sink or heat spreader in a semiconductor package and expose a surface of the heat sink to outside of an encapsulant that encapsulates a chip mounted in the semiconductor package by which heat produced from the chip can be directly dissipated via the exposed surface of the heat sink to the atmosphere, thereby effectively enhancing heat dissipating efficiency. Such a package structure with an exposed heat sink is taught by U.S. Pat. Nos. 5,796,159, 5,598,034 and 5,608,267 in which the heat sink is attached by means of an adhesive to and below a die pad or leads of a lead frame.
However, the foregoing attachment between the heat sink and the lead frame via the adhesive would undesirably make fabrication processes more complex and cost-ineffective to implement. Accordingly, U.S. Pat. Nos. 5,328,870, 5,381,042, 5,444,602, 5,489,801 and 5,530,295 disclose a semiconductor package not having to adhere a heat sink to a lead frame; this type of package structure is named EDHS-QFP (exposed drop-in heat sink-quad flat package) generally fabricated by procedural steps illustrated in FIGS. 4A to 4D. Referring to FIG. 4A, an encapsulation mold 20 is prepared for use in a molding process and composed of an upper mold 21 and a lower mold 22, each of the upper and lower molds 21, 22 having a cavity 210, 220 respectively. Then, a heat sink 23 is placed in a drop-in manner into the cavity 220 of the lower mold 22, with a bottom surface 230 of the heat sink 23 abutting against a bottom wall of the cavity 220 of the lower mold 22.
Referring to FIG. 4B, a die-bonded and wire-bonded lead frame 24 is stacked on the heat sink 23. A chip 25 is mounted on a surface of a die pad 240 of the lead frame 24 and electrically connected to inner leads 241 of the lead frame 24 via a plurality of bonding wires 26, allowing a surface of the die pad 240, opposite to the surface mounted with the chip 25, to be in contact with a top surface 231 of the heat sink 23.
Subsequently, referring to FIG. 4C, when a mold engaging process is performed to engage the upper mold 21 with the lower mold 22, outer leads 242 of the lead frame 24 are clamped between the upper and lower molds 21, 22, and the chip 25 and bonding wires 26 formed on the lead frame 24 are received within the cavity 210 of the upper mold 21. With the upper mold 21 being coupled to the lower mold 22, a resin injecting process is performed by which a resin material such as epoxy resin is injected into the cavities 210, 220 of the upper and lower molds 21, 22 to form an encapsulant 27 that hermetically encapsulates the chip 25, the bonding wires 26, the heat sink 23, and the die pad 240 and inner leads 241 of the lead frame 24, allowing these encapsulated elements to be protected against damage from external moisture or contaminant. As there is no adhesive applied between the heat sink 23 and the die pad 240, during mold engagement, the die pad 240 and tie bars (not shown) connected thereto are adapted to provide a downward pressing force to press the die pad 240 toward the heat sink 23; in particular, the die pad 240 is usually spaced apart from the bottom wall of the cavity 220 of the lower mold 22 by a distance smaller than a thickness of the heat sink 23 by about 2-3 mils, whereby the tie bars are tensioned and rebound to allow the die pad 240 to abut against the heat sink 23. Such a method to first place the heat sink 23 in the cavity 220 and then stack the die pad 240 on the heat sink 23 without having to beforehand adhere the heat sink 23 to the die pad 240 via an adhesive can thereby simplify fabrication processes and reduce production costs.
Then, by removing the encapsulation mold 20 from the lead frame 24, the encapsulant 27 is completely formed on the lead frame 24, with the bottom surface 230 of the heat sink 23 and the outer leads 242 being exposed to outside of the encapsulant 27, as shown in FIG. 4D. The exposed heat sink 23 facilitates dissipation of heat produced from the chip 25, and the exposed outer leads 242 can be bent or deformed to be input/output (I/O) ports of the semiconductor package, which are electrically connected to an external device such as printed circuit board (PCB, not shown) so as to allow the chip 25 to operate via the external device.
However, the above EDHS-QFP structure still renders significant drawbacks. As no adhesive is applied between the heat sink and the die pad, such an interface may be considered delaminated and undesirably forms a gap between the heat sink 23 and the die pad 240. This gap is normally small and not permeable for the resin material used to fabricate the encapsulant 27, as shown in FIG. 5A, thereby forming a delaminated interface between the heat sink 23 and the die pad 240 with air residing in the gap. Since air has poor thermal conductivity of 0.024 W/Mxc2x0 C., heat produced from the chip 25 and transmitted through the die pad 240, the gap (air), and the heat sink 23 would undesirably increase thermal transfer resistance and degrade heat dissipating efficiency.
On the other hand, if the small gap between the heat sink 23 and the die pad 240 may be partly filled with the resin material and leaves a void 28 formed in the middle of the gap, as shown in FIG. 5B; as a result, a central portion of the die pad 240 lacks support and the chip 25 may easily crack (as indicated by arrows in the drawing) due to impact from a flow of the resin material especially in a step of building up a packing pressure of the molding process, thereby seriously damaging quality and yield of fabricated products. This chip-cracking situation is more severe for relatively thinner chips. As chips are developed toward high integration and low profile, the larger a chip, the thinner the chip is (for example, even having a thickness smaller than 10 mils), and correspondingly a larger die pad is required, which makes it even harder to completely fill the gap between the heat sink and the die pad and thereby easily leads to void-induced chip cracks. Besides, similarly, air of poor thermal conductivity residing in the void 28 may increase thermal transfer resistance and degrade heat dissipating efficiency as to transmit heat from the chip 25 to the heat sink 23 for dissipation.
Therefore, the problem to be solved herein is to provide a semiconductor package which can prevent chip cracks, improve heat dissipating efficiency and reduce fabrication costs thereof.
An objective of the present invention is to provide a semiconductor package with a heat dissipating structure in which a resin material forms a supporting member between a die pad of a lead frame and a heat sink to provide proper support for the die pad, so as to prevent cracks of a chip mounted on the die pad during a molding process, thereby assuring yield and reliability of fabricated products.
Another objective of the invention is to provide a semiconductor package with a heat dissipating structure in which a bottom surface of a heat sink is exposed to outside of an encapsulant used for encapsulating a chip incorporated in the semiconductor package without forming a delaminated interface or void between the heat sink and a die pad wherein the chip is mounted, thereby improving heat dissipating efficiency of the overall package structure.
A further objective of the invention is to provide a semiconductor package with a heat dissipating structure in which a die pad is directly stacked on a heat sink without having to use an adhesive for adhering the heat sink to the die pad, thereby reducing production costs and simplifying fabrication processes.
In accordance with the foregoing and other objectives, the present invention proposes a semiconductor package with a heat dissipating structure, comprising: a lead frame with a die pad, the die pad having an upper surface and a lower surface opposed to the upper surface; at least a chip mounted on the upper surface of the die pad and electrically connected to the lead frame; a heat sink having a top surface and a bottom surface opposed to the top surface, the top surface being formed with at least a recessed portion of a predetermined depth, wherein the top surface of the heat sink comes into contact with the lower surface of the die pad; and an encapsulant for encapsulating the chip, the heat sink and part of the lead frame, wherein the recessed portion of the heat sink is filled with a resin material used for forming the encapsulant, and the bottom surface of the heat sink is exposed to outside of the encapsulant.
The above semiconductor package with a heat sink can yield significant benefits. A top surface of the heat sink is formed with at least a recessed portion having at least an opening at an edge of the heat sink, and the opening is directed in parallel to a flow direction of a resin material used for forming an encapsulant that encapsulates a chip mounted on a die pad of a lead frame. During a molding process, the die pad of the lead frame is stacked on the top surface of the heat sink with the recessed portion being disposed between the die pad and the heat sink, whereby the recessed portion enlarges a gap between the heat sink and the die pad and facilitates the resin material to smoothly fill into the gap and the recessed portion via the opening of the recessed portion to form a supporting member between the die pad and the heat sink thereby without leaving a delaminated interface or void between the die pad and the heat sink. With part of the recessed portion of the heat sink corresponding in position to a central portion of the die pad, the supporting member can provide proper support for the central portion of the die pad, so as to prevent the chip mounted on the die pad from cracking in a step of building up a packing pressure of the molding process, thereby assuring yield and reliability of fabricated products. Since the gap between the heat sink and the die pad is filled with the resin material having thermal conductivity of 0.8 W/Mxc2x0 C. significantly greater than that of air (0.024 W/Mxc2x0 C.), heat produced from the chip can be more efficiently transmitted through the die pad, the gap (resin material), and the heat sink for dissipation as compared to the prior art in which a thermal transfer path includes air residing in a delaminated interface or void between the die pad and the heat sink; therefore, the proposed semiconductor package desirably reduces thermal transfer resistance and enhances heat dissipating efficiency. Moreover, a bottom surface of the heat sink is exposed to outside of the encapsulant and helps dissipate the heat from the chip, thereby further improving the heat dissipating efficiency of the semiconductor package. In addition, the die pad of the lead frame is directly stacked on the heat sink without having to use an adhesive for adhering the heat sink to the die pad, thereby reducing production costs and simplifying fabrication processes.